Telephone or like system



Aug. 28, 1945. c. GlLLlNGS ET AL 2,383,541

TELEPHONE OR LIKE SYSTEM Filed Dec. 51, 1945 '7 Sheets-Sheet 1 SUB 0RS1 VF [R51 151 MAZ Fi iO INVENTORS CHARLES GILLINGS CHARLES EDMUND BEALE ATTORN EY Aug. 28, 1945. c. GILLINGS ET AL 2,383,541

TELEPHONE OR LIKE SYSTEM Filed Dec. 31, 1945 '7 Sheets-Sheet 2 5A4 Fig. 6

4 5 F INVENTOR CHARLES GILLINGS CHARLES EDMUND BEALE ATTORNEY Aug. 28, 1945- c. GlLLlNGS ET AL TELEPHONE OR LIKE SYSTEM Filed Dec. 51, 1943 7 Sheets-Sheet 3 cs1\ cos VFT1 INVENTORS CHARLES GILLINGS CHARLES EDMUND BEALE ATTORNEY Aug. 28, 1945. c. GILLINGS ET AL TELEPHONE OR LIKE SYSTEM Filed Dec. 31, 1943 '7 Sheets-Sheet 5 W M NO MT. A fi A q 05 v 5M 1 I MD M. s ww m H M L2 1 2 m h m melll/flo PM 1 d m m L B 1 o 4 C 2 INVENTORS CHARLES GI LINGS CHARLES EDMUND BEALE BY 6 Z ATTORNEY A g- 28, 9 c. GILLINGS ET AL 2,383,541

TELEPHONE OR LIKE SYSTEM I 7 Sheets-Sheet 6 Filed Dec. 51, 1943 N1 m WR 6 6X1 CR2 NNZ H Car E- INVENTORS CHARLES GlLLlNGS CHARLES EDMUND BEALE ATTORNEY Aug. 28, 1945. c. GlLLlNGS ET AL TELEPHONE OR LIKE SYSTEM Filed Dec. 51, 1945 "r sheets-sheet 7 MRB MRC P INVENTORS CHARLES GILLINGS crux LES zommo BEALE ATTORNEY 7 arranged for voice frequency working Patented Aug. 28, 1945 2,383,541 TELEPHONE OR LIKE SYSTEM Charles Gillings and Charles Edmund Beale, Livignors to Automatic Elecerpool, England, ass

trlc Laboratories Inc.,

tion of Delaware Chicago, 111., a corp ra- Application December 31, 1943, Serial No. 516,322 In Great Britain February 13, 1943 12 Claims.

The present invention relates to telephone or like signalling systems and is more particularly concerned with the setting and control of automatic telephone switching apparatus over long trunk routes involving mixed systems of voice frequency and direct current dialling and supervision. The invention may be considered as a development of the arrangements disclosed in applications Serial Nos. 502,884 and 502,885 filed Sept. 18, 1943, and its general object is to improve the operation of systems of this type by cutting out unnecessary repetitions of impulses in the direct current portions of the connection so that operation may be effected at higher speeds and with less liability of error due to distortion.

In the earlier of the above-mentioned specifications it is suggested that if the connection which is to be set up includes a direct current link following a voice frequency link the avoidance of two repetitions in the same exchange may be effected by giving access to the outgoing auto-to-auto repeater on the line side so that this repeater is in effect by-passed for connections of this type. Such an arrangement, however, is not altogether desirable since it involves conditions representing a departure from standard practice and moreover it cannot be applied satisfactorily if a direct current signalling portion is involved at a further exchange in which case impulse repetition is unavoidable.

According to one feature of the present invention, in a telephone system employing voice frequency currents for supervisory signalling and for controlling the selective setting of automatic switches, an incoming repeater for converting incoming voice frequency signals into direct current signals is arranged to transmit a voice frequency testing signal forward at the end of each train of direct current impulses and to respond to a characteristic signal sent back from an outgoing repeater if the succeeding inter-exchange trunk is operated on a voice frequency basis and thereupon to disable the converting equipment and condition the repeater for straight-through transmission of the voice frequency currents.

According to another feature of the invention, in a telephone system employing voice frequency currents for supervisory signalling and for controlling the selective setting of automatic switches, each time an inter-exchange trunk is taken into use a repeater at the outgoing end thereof is arranged to test for a signal transmitted from a preceding stage of the connection and in response to such signal to transmit a characteristic signal quency currents.

back to said preceding stage and to disable the equipment for converting direct current signals into voice frequency signals so as to permit straight-through transmission of the voice fre- Such straight-through transmission avoids the distortion which is otherwise introduced at each repetition of direct current impulses and permits of more rapid operation in response to super-' visory signals which can be transmitted from end to end in unchanged voice frequency form.

Such an arrangement, however, cannot be used if the direct current links employ impulse regenerators and as a number of these devices are already in use some considerable time is likely to elapse before it will be possible to be sure that no connection which can be set up from a particular exchange shall involve a regenerator. It is clear that if regenerators are included, endto-end impulse transmission is not feasible because of the delay introduced by the regenerators which split the line during the sending out of stored digital information. In other words, it is not possible to switch through and signal direct with voice frequency currents without involving a delay period between transmitted digits which would be inadmissible since it is impossible to know precisely what state of operation the regenerators have reached until sufficient time has elapsed for them to clear out all stored digits. In these circumstances after the connection has been set up, special precautions have to a be taken with the supervisory signals to ensure that these shall be passed from section to section in cascade and that no signal can operate the responding equipment of a plurality of sections simultaneously as this would cause overlapping of signals and confusion as to the information it was intended to convey. This particularly applies to the called subscriber answer signal in response to which an acknowledgment signal is usually adapted to be transmitted forward from the receiving point.

According to another feature of the invention, therefore, in a telephone system employing voice frequency currents for supervisory signalling and for controlling the selective setting of automatic switches, arrangements are provided whereby supervisory signals transmitted from one exchange to another over an inter-exchange trunk line are either prevented from extending directly to further exchanges or are enabled to do so depending on the conditions prevailing for the connection thus far built up so that as required supervisory signals may be transmitted from one end of the connection to the other either in cascade or direct.

Although cascade transmission of supervisory signals on connections involving regenerators provides a satisfactory basis of operation, it will be appreciated that end-to-end transmission is more to be desired in view of the resulting saving in time. Now when the subscriber answer signal is given all the regenerators will have finished their operations and hence the basic objections to end-to-end transmission no longer apply.

It is therefore arranged according to a. further feature of the invention that in a telephone system employing voice frequency currents for supervisory signalling and for controlling the selective setting of automatic switches, said system including in tandem at least three exchanges and two inter-exchange trunk lines over which signalling is efiected by voice frequency currents, signalling is initially efi'ected from exchange to exchange in cascade by way of repeaters for converting voice frequency signals into direct current signals and vice versa and in response to the reply of the wanted party the converting equipment is disabled to permit the straight-through transmission of voice frequency currents constituting subsequent supervisory signals.

The invention will be better understood from the following description of one method of carrying it into effect, reference being had to the accompanying drawings comprising Figs. 1 to 10.

Of these, Fig. 1 is a trunking diagram showing a typical connection to which the invention may be applied. Figs. 2-5 when arranged in the manner shown in Fig; 6 show a circuit diagram of an outgoing voice frequency (V. F.) relay set which may be seized either from an operator's position or from a selector level, while Figs. 7 and 8 when arranged with Fig. 7 on the left show a circuit diagram of an'incoming relay set which terminates an inter-exchange V. F. trunk line and which has associated therewith an incoming selector. The outgoing relay set is assumed to operate on the known sleeve-controlled basis. Figs. 9 and 10 show circuit modifications which when applied respectively to the outgoing and incoming V. F. relay sets enable end-to-end signalling to be introduced for certain supervisory signals on connections involving direct current (D. C.) impulse regenerators.

The circuits of the outgoing and incoming V. F. relay sets are very similar to those disclosed in the two prior specifications already mentioned, some of the apparent differences resulting from the fact that in the present circuits the conventlons, nomenclature and symbols have been brought more nearly into line with the present day practice of the British Post Oflice.

Referring first to Fig. 1, it will be assumed that a connection is to be set up from the manual board MB in exchange A to a subscriber in exchange D. when the operator plugs into the outgoing V. F. relay set ORBI, a. V. F. seizing signal is sent over the V. F. trunk VFII to exchange 3 to prepare the incoming V. F. relay setlRSl for operation.

When the operator dials the first digit, the impulses are converted at relay set ORSI into suitable V. F. coded signals which are received by the incoming relay set IRS! and are there converted back into D. C. loop impulses which operate the incoming selector ISI. This then seizes a D. C. trunk line DCT to exchange C by way of the auto-to-auto repeater A-A REP since it is as- 7 sumed that direct current signalling is employed between exchanges B and C.

The second digit dialled by the operator is transmitted in coded V. F. form from exchange A to exchange B where it is converted into D. C. impulses which are repeated by the auto-to-auto repeater over the trunk line DCT to incoming selector IS! in exchange C.

The trunk line VFI'I from exchange C to ex change D is arranged for V. F. operation and it is therefore no longer essential for the exchange B incoming relay set IRSI to convert the third and any subsequent digital V. F. codes into trains of D. C. impulses as these V. F. codes could satisfactorily be transmitted straight through from the originating relay set ORSI to the exchange D terminating relay set IRS2. The determination of when the second V. F. portion is reached is eil'ected by arranging for the incoming relay set IRSI to transmit a comparatively long V. F. signal, of the order of 800 milli-seconds (m. s.) after each train of direct current impulses. This V. F. signal pulse is without effect until the subsequent outgoing V. F. relay set ORS2 is reached and this relay set on receiving the pulse returns a battery reversal signal back over the line which signal on being received at the relay set IRSI initiates through switching conditions thereat. The V. F. signal pulse also initiates through switching conditions at the outgoing relay set ORSZ so that the third and any subsequent digits which are converted into coded V. F. form for transmission from exchange A to exchange B are transmitted without conversion into D. C. form through exchanges'B and C to the terminating exchange D where they operate the incoming relay set IRBI. This converts them into D. C. pulses for operating the appropriate switch or switch train therein to gain access to the required subscriber.

In exchange B SUB designatm a local subscriber, while the small cross indicates the line switch or line finder :by means of which access is gained to a local numerical switch with its bank multiple to the bank of the incoming selector ISI.

As regards the circuit of Figs. 2-5 corresponding to relay set ORSI or OBS! in Fig. 1. this is largely identical with that disclosed in the second above-mentioned specification but is modifled in several important respects. These modifications comprise the addition of a relay RB (Fig. 5), the alteration of the circuit of relay AC (Fig. 4) which previously functioned to effect suitable discrimination in response to seizure of the relay set from a selector level and the provision of means for responding to a signal of Y frequency transmitted forward.

The general method of operation is the same and the distant incoming equipment is conditioned for response to each coded digit by sending a coded prepare signal consisting of two frequencies only which diifer according as the digit to be transmitted is in the group l-5 or 6-0 A typical code on this basis is set out below and this is assumed to be employed in the example which follows.

Dlgit Prepare Code Digit Prepare Code 1 WY W W2 2. WY X WZ l WY Y WZ Z 4. WY WX WZ WX 5 WY XY WZ XZ The coded prepare pulse is assumed to have a duration of 160 m. s. and the following coded digit pulse a duration of 100 m. s.

The V. F. supervisory code to be employed is Considering now the circuit operations, the

description which follows will only enter into detail when circuit operations are concerned distinct from those of the prior specifications reierred to.

Assuming first that the outgoing relay set is seized from an operator's position, say manual board MB at exchange A, Fig. 1, when.the operator plugs into jack JCK, relays M, Fig. 2, and MM, Fig. operate in turn,'whereupon her supervisory lamp will flicker due to the connection of flicker earth over lead l0, Fig. 2, to the sleeve circuit via the upper low resistance winding of relay M. Relay MM in operating brings up relay SR, Fig. 5, by way of its upper winding and this causes switch TS to advance its wipers to position 1 where relays CO and FYX are operated. Relay .CO in operating brings up relay BS, Fig. 3, which is without efiect. Relay FXY brings brings up relay CS and causes switch TS to self-drive to position 3, whereupon relay SR releases and relays FXY and CS releases in turn. During this time contacts CSI and CS2 and FXYl, Fig. 3, will have caused a 100 m. s. pulse of X frequency to be transmitted forward over the trunk line to the incoming relay set IRS! at exchange B to bring about seizure of the associated incoming selector ISI. Relay FXY on releasing brings up relay FC which locks up and causes switch TS to advance to position 4 from where it advances to position 6 under control of the interrupted earth source connected to lead it which connects with the switch magnet over bank and wiper TS5.

When the operator throws her dialling key, battery is connected at her position circuit to the speaking leads and relay RR, Fig. 2 is operated over its upper winding and high resistance is introduced at the position circuit into the sleeve circuit to extinguish the calling supervisory lamp. Relay RR eflects the operation of relays A, Fig. 2,-B, Fig. 4, and BR, Fig. 5, in turn and the latter. relay disconnects relay RR and connects relay A to the jack tip and ring conductors.

When the operator dials the required subscribers number, the impulses are received on relay A which repeats them via relay C, Fig. 4, to the driving magnet DSAM of the first digit storing switch DSA which steps its wipers to the corresponding position. Relays B and C hold operated during the impulsing and the latter operates relay CS to prepare the V. F. signalling circuit at contacts CSI and CS2. Relay C also energises the driving magnet DDM oi the digit distributor switch DD but the wipers of this switch do not move until the magnet is tie-energised.

At the end of the first series of impulses relay A holds and relay C releases after its slow period thus operating relays CNA and CNR in series over cwiper DSAI If the digit dialled is 5 orless, relay CNA operates over its left-hand winding and as will appear subsequently an interdigital pause is provided between successive re-transmitted digits which is shorter than that provided for digits 6-0. Relay CNA in operating at contacts CNAI and CNA2 extends earth over wiper and bank DSAI and bank and wiper S03 to advance the sending control switch SC from its home osition to position 1 where relay ST is operated. Relay ST connects up its low resistance right-hand winding to self-drive switch SC to position 4, maintains relay CS and at contacts ST! and 8T3, Fig. 3, causes a prepare pulse of WY frequency to be connected to the trunk line extending to exchange B. when switch SC reaches position 4, relay ST releases after its slow period to disconnect'the WY prepare pulse and to operate relay S. The duration of the prepare pulse is measured by the time taken for switch SC to step to position 4 plus the slow release period of relay ST which gives a total figure of 160 m. s.

Relay 8 holds relay CS and connects the digit code pulse to line, this pulse being dependent for its composition on the setting of the digit switch Wiper DSA3 or DSAl, Fig. 3, and is thus characteristic of the digit dialled. Relay S short-circuits its left-hand high resistance winding to self drive switch-SC to position 8 where relay Z operates over its lower high resistance winding (magnet SCM being non-operative in this condition) and at contacts Z3, Fig. 3, disconnects the digit code pulse from the trunk line. Relay S also releases but relay Z holds relay CS. 1 The duration of the code pulse is measured by the time taken for switch SC to step from position 4 to position 8 which time, together with the operating time of relay Z approximates to 100 m. s. Relay Z at contacts Z2 holds relays CNA and CNR and at contacts Zl extends earth forward via its upper low resistance winding to home switch DSA whereupon relay Z releases and releases relay CS and. causes switch SC also to drive to its home position.

If in the meantime the operator has dialled a second digit, say 4, which will be received on switch DSB (not shown) relay CNR will be maintained operated in series with relay CNB (not shown), relay CNA having released on the release of relay Z. Accordingly,-when switch SC arrives at its home position it will again selfdrive to position 1 where relay ST will re-operate to send out the prepare pulse of WY frequency iollowed by the code pulse WX for digit 4 which is picked up from the setting of switch DSB.

If, as assumed, the first digit was 5 or less, the interdigital pause between the two code digits transmitted to line will be measured by the heming time of switch DSA plus the homing time of the 50-point switch SC giving a total of not less than 1200 m. 5. During this period live or less machine-generated impulses are delivered by the distant incoming relay set to the incoming selector, then hunting takes place to select an idle trunk in the level selected, after which an 800 In, a. pulse of Y frequency is transmitted forward from the relay set as will be later described. While the selector is hunting, the incoming relay set decoding relays are released and reset to receive the second prepare and code signals which will require a time period of 160 plus m. s. giving a total of 260 m. s. which addd to the pause period of the order of 1200 m. s. already mentioned, gives a total the complete interdigital pause.

Ii the first digit had been 6-0, a longer interdigital pause would be required to enable the incoming selector to perform its sequence 01' operations and this will be described in relation to the change of prepar signal which must also take place to characterise adigit in the second group. Under these conditions relay CNA will be operated over its right-hand winding in series with relay CNR and at contacts CNA3 brings about the operation 01' relay HS which locks over contacts HSI. Relay HS at contacts HS'I, Fig. 3, changes over the connections of the prepare pulse circuit so that when relay ST operates on the first step of switch S0, a WZ pulse is connected to line. This is followed by the coded digit pulse and when relay Z operates to terminate the pulse,

. switch DSA drives to its home position thus measuring oilv part of the interdigital pause, Relay HS is then disconnected at wiper and bank DSAI but holds for a short period due t its slug and at contacts HS3 causes switch DSA to make another half revolution. During this latter operat'ion relay HS is released but relay Z remains held over its upper winding and thus brings about a substantial increase in the interdigital pause. This extra pause ensures that the distant equipment has sufilcient time to perform the operations already mentioned before the next decoded digit is transmitted.

When all stored digits have been transmitted in code, no further kick-on circuit will be completed for the sending control switch SC and all the relays CNA-CND and relay CNR will also be restored to normal. Relays M, MM, CO, BS, FC, A, B and BR remain operated, switch TS remains in position 6 and switch DD occupies a position in accordance with the number of digits dialled.

When the operator restores her dialling key, the current in the sleeve circuit is increased and the battery applied to the tip and ring conductors is removed shortly afterwards. Relay LR, Fig. 2, thereupon operates and prepares the supervisory circuits and releases relays A, B and BR in turn. Relay B in releasing homes the distributor switch DD (Fig. 4) to its mid-position ready for the operation of relay AA at a. later period, while relay BR in releasing connects the low resistance lower winding of relay RR into the sleeve circuit, relay RR being non-operative but causing the operator's supervisory lamp to glow. Relays M, MM, CO, BS, FC and LR remain operated and the line condition is such that the operator is able to receive tones corresponding to the condition of the wanted party's line and, if necessary, speak without the necessity for a called party answer signal.

When the called party replies, an answer signal comprising 140 m. s. pulses of Y frequency at spacings of 360 m. s. is transmitted back from the incoming relay set, this signal being repeated until acknowledged signal from the outgoing relay set. Upon reception of the Y frequency, relay Y in the VF receiver VFR, Fig. 3, operates and opens the circuit for relay BS. On the release of relay BS after aperlod of the order of 100 m. s. relays MS and MT are operated in turn and hold for the remainder of the Y pulse. When the Y pulse is finished relay BS re-operates and further opens the circuit for relay MS which has a release lag of the order of 350 m. s. If the interval between received pulses is of the correct duration, namely 01 the order of 1460 m. s. for

by the transmission of an X.

360 m. s., relay MS will release but not relay MT, since a period of 360 m. s. plus the m. s. release time of relay BS will elapse before any further circuit can be completed to relay MS so that on reception of the next Y pulse, relay SA will be operatedand at the end of this pulse relay SB will operate in series with relay SA. Relay SA in operating gives local and through supervision at the outgoing operators position. Relay SB brings up relay AA, Fig. 4, in series with the DD switch magnet DDM which proceeds to self-drive, while relay AA re-operates relay CS, Fig. 5, and at contacts AAI, Fig. 3, applies X frequency to the outgoing trunk. The duration of the X frequency acknowledgment signal is measured by the homing time of switch DD plus the release time of relay AA. when the X pulse is received at the incoming relay set, transmission of the Y pulse is stopped. Relay BS therefore remains operated, while relay MT releases.

Conversation now proceeds, and when the called party clears, the incoming relay set transmits Y pulses of the same type as for the called party answer signal. At the outgoing end relays M, MM, LR, CO, FC, BS, SA and SB are held switch TS is in position 6 so that the first m. s. pulse of Y frequency releases relay BS and operates relays MS and M71- as before. The following 360 m. s. break period operates relay BS and releases relay MS but not MT so that when the next Y pulse is received relay BS again releases and during the slow release period of relay MT relay SY now operates. Relay SY which has a release lag of 400-450 m. s. lock up to the Y pulses, operates relay SZ and releases relay SA which gives the necessary clearing supervision at the outgoing operator's position.

When the operator clears by removing the plug from the jack, relays M, MM and LR release in urn.

Relay CS now re-operates via bank TSI, whereupon X frequency is applied to the trunk line. With relay SB operated the switch TS self-drives via bank TS3 from position 7 .to position 15 and then steps by means of interrupted earth on lead II and via bank TSS to position l8. Relay m is then operated via bank TSl to disconnect the X frequency, the period of application being of the order of two seconds, while it also connects up Y frequency to the trunk line. Wiper 184 on being stepped on to position 19 causes relays SR and BR to release after their and on release of relay BR earth is extended over bank T83 to step the switch from position 19 so that relay FXY releases and after its slow release its home position where relays CO, FC, and SB release. During the forward application of Y frequency relay BS will have released and relays MS and MT operated but without usefollows:

where'u n at, contacts to the failure of the called subscriber to reply or if busy or NU tones are received, the clearing .pulse of X frequency is lengthened to 6 seconds,

owing to the non-operation of relay SB, in order of 'aselector taken into use by a subscriber or [from a selector level following a completely D. C.

signalling link, in which case no V. F. signal pulse Lisreceived on seizure; or that the relay set is taken into use from a selector level following a circuit which includes a V. F. link and also a D. C.

link involving an impulse regenerator, in which case any V. F. signal pulse fed forward will not be able to, get-through owing to the splitting of the line by the regenerator during its operation, in both these cases the operations are substantially as .already described and may be summarised as Seizur oflthe relay set is in this instance effected by earth extended forward over the private conductor P and relay SR thereupon operates andmoves the switch TS off normal as previously described and locks up over its upper winding to contacts] and 2 of bank TSfi. The TS switch in advancing to position I operates relays 00, BS,

CSiand FXY. Relay SR in operating at contacts SR6 andSBI, F 3, connects the V. F. receiver responsive to-Y frequency to the line conductors fromthe selector level by way of. con en s- QC. nd eifect in this: instance. tended forwardvia the selector and this operates This operation is without A loop will also be exrelay A which in turn brings up relays B and BR, BB2 a guard earth is extended back over the P lead to hold the selector.

Relay FXY extends a 100 m. s. X frequency seizing pulse forwardly over the trunk to the distant incoming relay set and at the same time causes the switch TS to self-drive to position 3 where relaysSR and FXY are disconnected and relay FXY after itsslow release period terminates the X pulse and releases relay CS. Relay FC, Fig. 5, is then operated over bank TSl and locks up and serves toj step switch TS to position 4 from which it slow steps frominterrupted earth on lead I l to position, 6. this point the call proceeds as for an operator call as already described, that is tosay, relay A responds to the dialled impulses which are stored via the distributor switch DD on switches f DSA .DSD and are subsequently sent out under control of the sending'switch SC in codedV. F. form- When the called party answer signal is'subsequently received from the distant incoming end, relay SA in operating reverses the A relay connections towards the calling party for supervision, while relay SB causes an x pulse acknowledgment .signal to be transmitted forweraly tothe distant exchange to terminate the backward Y answer signal and so enable conversational conditions to be set up.

If. however, the relay set is seizedfrom a circult including a V. F. link but not including a regenerator, for example if the relay set concernedis 01252 at exchange C, Fig. l, and assuming that A- A REP in exchange B is an ordinary impulse repeater then on seizure from the selector, earth will be extended forward over the P switch TS of! normal as previously described and locks up over its upper winding to contacts I and 2 of bank TS. The TS switch in advancing to position 1 operates relays C0, BS, CS and FXY. Relay SR in operating connects the V. F. receiver VFR to the line conductors from the selector level, contacts FXY5 and FXYG serving for contact wetting purposes. A loop will also be extended forward via the selector and this operates relays A, B and BR in turn. Relay FXY extends a m. s. X frequency seizing pulse forwardly over the trunk to the distant incoming relay set IRS! at exchange D, and at the same time causes the switch TS to self-drive to position 3 where relays SR, FXY and CS are in turn released, relay FXY after its slow release period terminating the X pulse. Before the release of relay FXY however, a prolonged pulse of Y frequency, of the order of 800 m. s. will be projected forward from the incoming relay set IRS! in exchange B, such a Y frequency pulse following each direct current impulse train from relay set IRS! but being ineffective until an outgoing relay set is reached. Relay Y in the V. F. receiver VFR- therefore operates and hence relays MS and MT are energised. Relay RB, Fig. 5, now operates and in turn brings up relay AC, Fig. 4,.from the earthed contacts Bl. Relay RB in operating at its contacts BB2 and R33, Fig. 2, reverses the connection of relay A to the incoming line and, as will be subsequently described in connection with Figs. '7 and 8, sets up through switching conditions in the exchange B incoming relay set IRSi so that subsequent V. F. digital information received by this relay set will not be converted to D. C. loop impulse form but will be extended straight through to exchange C. Relay AG in operating locks up at contacts AC i, at contacts AC3, Fig. 5 completes a self-driving circuit for switch TS to advance it to position 20 and at contacts AC4 disconnects the interrupted earth lead I l and at contacts AC5, Fig. 3, disables the V. F. receiver VFR, whereupon relays MS and MT release. On the advancement of the switch TS to position 3 relay SR will have been disconnected at bank T84 and so releases relay RB but relay AC remains locked. The connections of relay A to line are therefore restored to normal, while on the release of relay SR, relay CS is also released and since relays BR and CO will still be operated, the incoming leads from the selector level will be extended straight through to the outgoing trunk via the repeating coil RPCI. With relays A, B, BR, CO, BS and AC operated and withswitch TS in position 20, further V. F. digital information transmitted from exchange A is extended straight through to exchange D without any conversion into D. C. impulses between exchanges Band C. At exchange D the incoming relay set IRSZ converts the received V. F. digital information into l). C. impulse train form to operate one or more automatic switches to give access to the wanted subscriber on this exchange. Subsequent supervisory signals are also transmitted in both directions on amend-to endi-Vrw. signalling basis.

Referring now to the incoming .relay set of Figs. '1 and 2, this is suitable for use as relay set conductor to operate relay SR which moves the 7 looks and brings up relay PY and at the same time extends a forward earth over the P conductor to the incoming selector ISI conductor to the selector, operates relay IL over the negative line in series with the selector impulse accepting relay and also operates relay Gx, whereupon relay PY is locked. Relays L, PY, NN, IL and GK are therefore held operated at this stage.

When the outgoing relay set ORSI at exchange A transmits the 160 m. s. prepare pulse of WY or WZ frequency as described, relays IW and |Y or IW and IZ operate in receiver IVFR. of the incoming relay set and bring up their relief relays WR and YR or WR. and ZR. -Depending upon which of the two prepare signals characteristic of the digits 1-5 or 6-0 is transmitted, relay CL alone is operated or relays CL and CH. In either case the line termination, normally comprising resistances YM and YN in parallel, Y? and Y8 and YT in parallel, the impedance of the repeating coil RPCI being sufficiently high to be neglected at this time, is converted into a loss pad with considerable attenuation which prevents the passage of V. F. signals beyond the incoming relay set.

The prepare pulse of 160 m. s. is followed by the code digit pulse of 100 m. s. duration, relay CL and relay CH if energised holding operated during the short interval between pulses due to their slugs. Assuming a first digit l, relay CL will be operated in response to prepare pulse WY over contacts YR2, WRf, 1R2, Ii and NNI to earth and prepares a circuit for relay CB which is at present short-circuited. When the prepare pulse is replacedby the code W for digit 1, relay- WR holds and relay YR releases, and relay CB therefore operates in series with relay CL which now holds. Relay CB in operating locks relay WR to store the digit and this relay over contacts WRA, Fig. 8, of a suitable pyramid of contacts prepares to connect earth to the contact 2 in the bank SS2 of the sending switch as in order to pear subsequently.

The contact pyramid of the code relays WR-ZR prepares to connect earth markings to the sender switch bank 832 to control the sending out of the digit concerned. The particular part of bank $32 which is to be effective at any time is determined in accordance with the digit concerned by the operated or normal condition of relay CH. If the digit concerned is .in the 1-6 group, relayCH will be normal and an earth marking will b prepared to one of the five contacts 26 via contacts SZI and CH2 and when the SS switch is set in motion the marking will be completed via bank, SS3 when wiper 88! reaches position 2, this wiper being earthed via contacts MA! and RN! to earth. If the digit concerned is in the 6-0 group, in which case relay CH will be operated, the earth marking will be prepared to contacts 7-11 of bank 882 via contacts 82 I, and will subsequently be completed via when wiper 88! reaches position '7. In the present instance where digit 1 is concerned, relay WR will be operated and relay CH will be normal so that an earth marking will be prepared to contact 2 of bank 882.

Returning to the operation. of relay CB in series with relay CL, on the operation of relay CB earth will be extended to operate relay CY over contacts 0L5, CBI, NR! and LI, while relay P will also be operated and will lock. Relay P at contacts P4 prepares a circuit for the line supervisory relay I, while relay CY in operating short-circuits its lower winding to render itself slow to release, at contacts CYI prepares a circuit for transmitting Y frequency forward and at contacts CY2 operates relay CC.

With relays CB and CC operated a circuit is completed for relay IG, Fig. 8, which operates when the impulse springs 66% M next open. Relay IG at contacts 163 drops relay 1L and transfers to the springs 33% M the forward selector seizing circuit previously closed by relay NR at contacts NNS. The springs 33% M are closed at this time since they are operated in the opposite manner to the magnet impulse springs 66% M which have just opened to bring about the operation of relay 16.

It will be noticed that the impulse springs 33% M are normally short-circuited over wiper S8! in its home position so that when the magnet springs 66% M next close to energise magnet SSMpf switch 88 the opening of springs 33% M is without effect. On the subsequent opening of springs 66% M, magnet 88M is de-energised and the SS switch wipers are thereupon advanced 'to position 1 where the springs 33% M now become eflective but are closed at this particular instant. On the next closure of springs 66% M, springs 33% M open to send out the first impulse and on the next opening of springs 66% M which causes the S8 switch wipers to advance to position 2 springs 33% M reclose to terminate the break portion of the first impulse. In this instance the digit concerned is 1 so that wiper 882 in position 2 will pick up an earth marking over operated contacts WRA and relay 82 will operate and at contacts 822 loops the outgoing line conductors to terminate the outgoing impulse train. At contacts SZI it completes a locking circuit for itself via bank and wiper S83 and at the same contacts removes the short-circuit from relay PR. which operates in series with relay 82 which holds. while at contacts 823 a self-driving circuit for magnet 88M via wiper and bank S84 is completed. Relay PR in operating at contacts PRI prepares for connecting the rectifier polarised relay DT across the outgoing line, at contacts PR2 completes an alternative holding circuit for relay CY, at contacts PR4, Fig. 7, releases relays CB and CL, whereupon relays WR and IG release, and at contacts PR8 connects up Y frequency to the transformer VFTI. Relay CB on releasing at contacts CB8, Fig. 8, connects the polarised relay DI across the outgoing conductors, resistance YV obviating any tendency of the selector to release during the changeover. and at operate terminate transmission of Y frequency and effect the through switching is sent back to the incoming relay set. The polarised relay DT which would respond to such signal does not therefore under these conditions.

when the SS switch in performing its selfdriving operation reaches position 12, relay SR operates in series with the magnet SM which cannot now operate. Relay SR short-circuits its high resistance left-hand winding and enables the switch to step to position 13. When this has occurred relay SR releases and at contacts SRI completes a circuit for advancing the switch to position 14. This operation continues until the switch reaches position 22 in which position relay PR is disconnected and releases to transmission of the Y frequency after a period of application of the order of 800 m. s. n release of relay PR, contacts PR3 complete a circuit to home switch SS and relay SZ then releases, whereupon relay IL re-operates over the negative lead. Relay PR in releasing also releases relay CY and this relay releases relay CC. Relays L, PY, NN, IL, GK and P remain operated.

0n reception of the next coded V. F. digit which may be assumed to be digit 6, relays CL and CH are initially operated and subsequently locked up in series with relay CB, while relay WR is again energised. Relay CB brings up relays CY and CC and relay IG then operates to initiate the sending out of the digit in D. C. impulse form. In this case, in consequence of the operation of relay CH no circuit is completed over contacts 2-6 of the switch SS and the earth marking is not therefore communicated to bank SS2 until the switch reaches position '7 when relay SZ operates and holds in series with relay PR which then operates. As before, relay PR in operating brings about the release of relays CB, CH, CL, WR and IG, connects relay DT across the outgoing leads, and applies the forward Y frequency pulse. Relay SZ self-drives the SS switch to position 12 from which it operates by interaction with relay SR towards position 22 to time the forward Y frequency pulse.

The second series of impulses will be repeated by the auto-to-auto repeater A-A REP, Fig. l, and will operate the incoming selector 182 in exchange 0 which will seize an outgoing relay set ORS2 having circuits as illustrated in Figs. 2-5. In consequence of the transmission of the Y frequency forward following the D. C. impulse train for digit 6, the relay set ORS2 willmomentarily reyerse battery back over the line to the auto-toauto repeater A-A REP at exchange B where it is repeated back to the incoming relay set and will bring ,up relay DT. Relay DT in operating locks up over its contacts DTI, at contacts DT2 shortcircuits relay PR, at contacts DT3 short-circuits relay SR and provides a quick homing circuit for switch SS, at contacts DTl, Fig. 7, locally operates relay IL and at contacts DTS brings up relay IR. Relay PR in releasing releases relays CY and CC in turn, whilerelay SZ also releases on the homing of the switch SS. Relay IR in operating at contacts IRI disables the loss. pad, at contacts IR2 disables the code signal responding circuit including relays CL, CH and CB so as to prevent further coded V. F. signals from being effective on the incoming relay set and at contacts IRS releases relay L.

Relays PY, NN, IL, GX, P, DT and IR remain operated and hence when the third and any subsequent coded V. F. signals are received, al-

though the relays WRZR will respond, the relays CL, CH and CB will be non-responsive and the loss pad will not be brought into circuit with the line so that the V. F. signals will be transmitted straight through via the repeating coil RPC2 to the outgoing side of the circuit whence they extend via the exchange B selector terminate the relay set IRSI a ISI and auto-to-auto repeater A-A REP and exchange 0 incoming selector 1S2 going relay set ORS2.

Relay set ORS2 will have responded to the Y frequency transmitted forward as already described in connection with Figs. 2-5 so that it oifers a straight-through path and hence the third and any subsequent coded V. F. signals are transmitted direct from the outgoing relay set ORSI in exchange A to the incomingrelay set IRS2 in exchange D. This relay set may be assumed to be identical with that shown in Figs. 7 and 8, and when it has converted the received V. F. codedinformation into direct current impulse form to actuate the selector to gain access to the wanted subscriber on this exchange, it will send forward in the same manner as for long pulse of Y frequency which is clearly inefiective so that relay DT therein is not operated. The relays L, PY, NN, IL, GK and P will therefore remain operated in relay set IRS2 while the required subscriber is being rung.

When the wanted party replies, relay I in relay set IRSZ at exchange D is operated over the positive lead and relay IL releases.. ..Relay I in operating brings up relay IR which disconnects relay L and on the release of this relay relay MA operates. Relay MA prepares a circuit for connecting'up Y frequency to the line transformer VFW, Fig. 7, connects the secondary of this transformer to the incoming trunk line, connects relay CY up to bank SSi, Fig. 8, and at contacts MA'I completes a self-driving circuit for switch SS. During the first seven steps of this switch, relay CY is operated from earth over wiper and bank SSI and therefore Y frequency is transmitted back over the connection set up, while during the remaining eighteen steps relay CY is not operated and hence with the normal adjustment of the switch to make fifty steps a second this will represent a Y frequency signal timing of m. s. on and 360 m. 5. off. So long as relay MA remains operated the switch continues to self-drive.

At the outgoing relay set where relays A, B, BR, CO,

ORS2 in exchange C BS and A0 are held operated at this time and the'switch TS is in position 20, the V. F. receiver responds to the Y frequency but owing to the fact that relay AC in this relay set is operated the energisation of the Y frequency relay produces no effect and the signal is' passed straight through. It also passes straight through the auto-to-auto repeater and the incoming relay set IRSI at exchange B where relays PY, NN, IL, GX, P, DT and IR are held operated dependent upon the outgoing relay set ORSI at exchange A. In this relay set relay AC is not operated, relays M, MM, CO, BS, FC and LR being operated and the switch TS being in position 6, and switch DD occupying its midposition so that the first pulse of Y frequency energises relays MS and MT after BS releases. During the succeeding blank period relay MS releases so that on reception of the next pulse relay SA is operated and at the end of this pulse relay SB operates in series with relay SA.

The operation of relay SA gives the operator to the outof the connection.

the usual answering supervision and relay SB energises relay AA and homes the digit distributor switch DD. During this time a 440 m. s. pulse of X frequency is sent forward and passes straight through the connection to the incoming relay set at exchange D where it operates relay XR. Relay GX is accordingly released owing to the length of time its circuit is broken by contacts XR4 while the operation of relay GY provides a locking circuit for relay PY. Relay PY releases at the end of the X pulse and in turn releases relay MA which thereupon terminates the trans- .mission of the interrupted Y pulse. Relay GX,

which re-operates at the end of the X pulse, at this time serves merely for guarding against false operation by parasitic X frequencies on the line.

The connection is now completely set up and conversation may take place, and it may be as well at this stage to review the circuit conditions obtaining in the various relay sets, which are as follows:

Relay set IRS2-relays NN, GX, P, I and IR operated.

Relay set ORSI-relays A, B, BR, C0, B8 and AC operated: switch T8 in position 20.

Relay set IRSl-relays PY, NN, GX, P, IL, DT

and IR operated.

Relay set RSlrelays M, MM, C0, B8, FC, LR, SA and SB operated; switch TS in position 6.

It may be pointed out that if the wanted party should reply immediately his bell commences to ring, it is conceivable that the battery reversal from the exchange D final selector might be received by the exchange D incoming relay set IRS2 before the termination of the Y frequency which is transmitted forward at the end of each outgoing impulse train. In this case it would operate relay DT while it was still connected up by relays PR and SZ. As previously described relay DT in operating short-circuits and releases relay PR, provides a quick homing circuit for switch SS, holds relay IL independently of the outgoing line, and brings up relay IR. Relay PR in re leasing releases relays CY and CC in turn while relay IR in operating sets up through switching conditions and releases relay L. On the release of relay SZ however, when the BS switch reaches its home position, relay I will be operated, since the battery reversal persists and relay I in operating opens the locking circuit of relay DT, whereupon relay IL is released and completes the circuit for relay MA to initiate the returning of the interrupted Y frequency subscriber answer. signal When the wanted connections to line tor are restored relay' IL is again operated, while relay I releases. Relay I in releasing at contacts II releases relay IR and recompletes an energising circuit for relay MA which was released in response to the acknowledgment signal from the originating end, and this relay in operating causes the transmission of interrupted Y frequency in the manner previously described, this signal now constituting a called sulbscriber clearing signal.

This is received by the outgoing relay set ORBi in exchange A and causes the operation of relays MS and MT as before, but since at this time relays SA and SB therein are already operated, relay SY is now energised and releases relay SA to give the operator clearing supervision.- Relay SY is sufliciently slow to remain operated the battery final selecparty hangs up. at the exchange D to normal and in relay set 1382 this relay set in lays together bring about the release and the latter releases Relay SR CS and these reextension of an X frequency signal forward over the connection time relays SY and SZ release. Since relay SB is operated, the switch TS continues to self-drive from position 7 to position 15 via bank T33 rupted earth to position 19 via 'bank T85. Relay P'X'Y is then operated over bank T84 and serves to disconnect the X frequency after a period of in operating re-operates relay the two second X pulse operates relays XR and circuit for relay N to the Y pulse and also connects earth to the P conductor extending forwardly to the automatic switch train in exchange D. relays YR and switch train whereupon the relay set is free for further calls. a

The two seconds X, 300 m. s. Y release pulse in passing through the intermediate incoming relay set IRBI in exchange Boperates the responding equipment to produce the release of the following manner. During YRand CR are operated and the latter translay DI and removes the earth from the P conductor extending to the 'auto-tc-auto repeater, the loop to this repeater having been opened on the release of relay NN. This interruption is repeated by the auto-to-auto repeater to the outgoing relay set ORSi in exchange C wh'ere relays A, B, BR, C0, B8 and AC are held operated and switch TS is in position 20. Relay A now releases and releases relays B and BR in turn. Relay AC then falls away and the switch TS is stepped to normal via banks TS! and T85, no signals being transmitted to line during this time. When the switch TS reaches normal, relays CO and BS release and th'e earth is removed from the incoming P lead to this relay set by bank and wiper T84, whereupon the preceding exchange C selector train is released and the whole connection is then completely broken down.

The circuit operations at an incoming relay set in circumstances where a short-circuited trunk is seized by an incoming selector, in which case relays SC and IG are brought into operation to prevent the short-circuit masking the V. F. receiver, will be appreciated from the similar operations described in application Serial No. 502,884, filed September 18, 1943.

The operations involved in cases where the originating operator clears during the setting up of a connection and where non-metered calls, such as a enquiry positions are set up, are also indicated in this prior specification.

The case will now be considered in which the connection includes one or more D. 0. links involving regenerators, for instance where. in Fig. l, the auto-to-auto repeater A--A REP includes a regenerator so that end-to-end impulse sending is not possible and in this case it is important to ensure that supervisory signalling from the called party takes place in cascade. Assuming that with a regenerator in the repeater A-A REP a connection has been set up as shown in Fig. i. this will mean that through signalling conditions will not have been set up in the relay sets IRS! and ORS2 so that in both the relay sets DB3! and ORSZ relay AC will'be normal and switch TS in position 6 and in both the relays sets IRSI and IRS2 relay DT will be normal:

Consequently when the called subscriber answers, the interrupted Y signal transmitted back from relay set IRS2 to ORS2 will operate relays MS, MT, SA and SB and relay MT in operating will bring up relay CS to open the signalling path and so prevent a sufficiently long pulse of the Y frequency signal passing further back to exchange B. Relay SB in operating initiates the transmission forward of the X frequency acknowledgment signal, the switch DD homing irom the positon it then occupies, while relay SA in operating reverses battery back over the D. C. link and this after repetition by A-A REP produces the operation of relay I and the release of relay IL in the relay set IRSI at exchange B.

At this relay setIRSI, relay I in operating brings up relay IR which disconnects relay L and on the releaseof this relay, relay MA opcrates to transmit interrupted Y frequency back to exchange A. At relay set IRSI the interrupted Y frequency signals are prevented from extending forward due to the operation of relay MA, while at the originating outgoing relay set ORSI they are prevented from passing further backwards by .virtue of the operation of relay CS. The outgoing relay set ORSI thereupon transmits forward the X frequency acknowledgment signal to terminate the sending out of the interrupted Y frequency from relay set IRSI.

The relays operated under the conversational conditions now established may be summarised as follows.

Relay set IRSZ-relays NN, GX, P, I and IR operated.

Relay set ORSZ-relays A, B, BR, 00, BS, FC,

SA and SB: switch TS in position 6.

Relay set IRSl-relays NN, GX, P, I and IR operated.

Relay set ORSl-relays M. MM, C0, BS, FC, LR, SA, and SB operated: switch TS in position 6.

Similar considerations apply when the called subscriber hangs up, in which case relay I in releasing in relay set IRS2 recompletes an energising circuit for relay MA, and this relay causes the transmission of interrupted Y frequency back to relay set ORS2.

At relay set ORS2 relay MS and MT operate, but since at this time relays SA and SB therein are already operated, relay SY is now energised and releases relay SA to restore the reversal extended to the incoming relay set IRSI when the called subscriber answered. Relay SY remains held to the interrupted Y frequencvy from relay set IRS2.

At relay set IRSl similar considerations apply to thatat relay set IRS2 and interrupted Y frequency is sent out to relay set ORSI, where relays MS, MT and SY are operated and relay SY is brought up and remains operated from the interrupted Y frequency signal from relay set IRSI.

When the operator clears, a two-second X frequency signal and a 300 m. s. Y frequency signal are sent forward to relay set IRSI where relays XR and GY operate and the former release relay GX which thereupon brings up relay CC and releases relays MA and CY to terminate the interrupted Y frequency signal being transmitted from this relay set. On the release of relay MA, the remainder of the two-second X pulse goes forward to the relay set ORS2 where it is without effect and proceeds to extend through to the relay set IRS! where relays XR and GY are operated, relay GX, released, relay CC operated and relays MA and CY released to terminate the Y frequency being sent back to relay set ORSZ. In response to the Y frequency portion of the operator clearing signal the relay sets IRS! and BS2 subsequently restore to normal in the manner already described and open the forward holding circuits to release the selector trains held operated therefrom. At exchange B the outgoing auto-to-auto repeater A-A REP thereupon opens the forward holding circuit on the D. C. trunks side so as to bring about release of the relay set ORS2 at exchange C. This relay set in releasing thereupon releases the incoming selector train at exchange C and all equipment is now restored to normal.

It will thus be seen that the clear-down signal from the operators position operates substantially simultaneously on a plurality of sections regardless of whether regenerators are involved in the connection or not, while when regenerators are involved in the connection the subscriber answer signal and subsequent supervisory signals suchas the subscriber clearing signal are transmitted on a cascade signalling basis. Although the above arrangements are satisfactory, it will be appreciated that the more supervisory signals that can be transmitted on an end-to-end basis the greater will be the saving of time. With this point in view modified circuit arrangements are proposed whereby the receipt of the subscriber answer signal at the originating end initiates the changeover to through V. F. signalling conditions so that subsequent supervisory signals such as the called subscriber clear signal shall be transmitted on an end-to-end basis.

For an understanding of this arrangement reference should be had to Figs. 9 and 10 which show the modifications which require to be made to the outgoing and incoming V. F. relay sets respectively.

From Fig. 9 it will be seen that the outgoing relay set V. F. receiver is arranged to respond selectively to both X and Y frequencies instead of only to Y frequency as before, while an additional circuit is provided for the through switchingarelay AC over extra contacts SAT, BS2 and AA From Fig. 10 it will contacts XR1 and XRB a through transmission circuit independently of the operated or normal condition of relay MA.

Referring again to Fig. 1 and reviewing the operations involved when the called subscriber answers on a connection involving a regenerator, when the answer signal after having been cascade transmitted is finally repeated in interrupted Y frequency form from the exchange B relay set IRSI back to the originating outgoing relay set ORSI, relay set ORSI thereupon transmits forward the X frequency acknowledgment signal to terminate the sending out of the interrupted Y frequency from relay set 1178i. With the relay se t ORSI modified in the manner shown in Fi 9 it will be seen that the associated V. F. receiver VFR can respond to the X frequency acknowledgment signal. but since relay AA is held operated during the homing of switch DD which times the x frequency acknowledgment signal, contacts AAI prevent the response of the V. F. receiver to the X frequency from being effective.

At the incoming relay set IRSI where relay IX in the V. F. receiver VFR comes up and brin s up relay X'R, relay XR is efl'ective in normal manner to terminate the sending out of the interrupted Y frequency to relay set ORSI by releasing relay MA but it will be noted from Fig. 10 that in the present instance the additional contacts KR! and X38 provide an immediate through path for the X frequency acknowledgment s gnal from relay set ORSI so that this signal, as well as being received in the incoming relay set IRSI, will also extend forward over the connection to the exchange C outgoing relay set ORSI. At this relay set the x relay in the V. F. receiver operates, and since by this time the X frequency acknowledgment from this .relay set to the terminat ng relay set IRS! will have been completely sent out, relay All will be normal and hence the V. F. receiver x relay is operating will bring up relay AC, relay SA being operated at this time, and relay B8, which serves for guarding against false operation by transient ,signals of X frequency, releasing some 100 m. s. after the energisation of the X relay. Relay AC in operating functions as before described to disable the V. F. receiver VFR as far as Y fr q be seen that additional are required to provide .in response to the reception of is concerned which is used for any subsequent supervisory nals such as the called subscriber clear signal. In addition .to being received at the outgoing relay set 0R8! the X frequency acknowledgment also extends forward to the incoming relay set IR82, but since this latter relay set will have already responded to an x frequency acknowledgment signal from relay set 0382 the receipt of a second acknowledgment signal will be without effect.

When subsequently the called subscriber clears, the interrupted Y frequency clearing signal is transmitted from relay set 11182. Since the V. 1". receiver of relay set 0R8! is disabled as far as Y frequency is concerned, this frequency will therefore extend right back over the connectionto the originating outgoing relay set ORSI where it functions as before described to give a clearins i nal to the operator. When the operator clears, the clear down of the equipment will be as already described for an end-to-end signalling connection.

What we claim Letters Patent is:

1. In a telephone system employing mixed systems of voice frequency and direct current signaling for controlling the selective setting of automatic switches, a repeater responsive to incoming voice frequency signals for transmitting corresponding direct current signals forward to succeeding switches, said repeater being operative to at times transmit a voice frequency testing signal forward and being operative in response to a consequent return signal to condition itself for straight-through transmission of subsequent voice frequency signals.

2. In a telephone system employing mixed systems of voice frequency and direct current signaling for supervision and for controlling the selective sett ng of automatic switches. a repeater for converting incoming voice frequency signals into corresponding outgoing direct current s gnals to control succeeding automatic switches. said repeater being operative to transmit a voice frequency testing signal forward after each direct current signal, a second repeater accessible to the succeeding automatic switches and operative said testing silnal to return a characteristic signal to said first repeater, said first repeater being operative in response to the return of said characteristic lilnal to condition itself for straight-through transmission of subsequent voice frequency signals.

3. In a telephone system, a plurality of exchanges. trunk lines connected therebetween, means in one of the exchanges for transmitting voice frequency signals over a trunk line to a second exchange, means in the second exchange responsive to said voice frequency signals for operating an automatic switch therein into association with a trunk line extending to a third exchange and for repeating subsequently received voice frequency signals as corresponding direct current signals to the third exchange, said second means being adapted to transmit a voice frequency testing signal forward following each direct current signal, an automatic switch in the third exchange operated in response to the direct current signals for further extending the connection, means in a succeeding stage of the connection responsive to said testing signal for as new and desire to secure by returning a characteristic signal to said second means, said second means being responsive to said characteristic signal to condition itself for straight-through transmission of voice frequency signals.

4. A telephone system employing mixed systems of voice frequency and direct current signaling for controlling the setting of automatic switches comprising means for transmitting voice frequency signals from a first exchange to a second exchange, and means in the second exchange for either extending said voice frequency signals directly to further exchanges or converting them into corresponding dir at current signals before they are extended depending upon the condition of the connection thus far built up in the further exchanges.

5. A telephone system includingat least three exchanges and two inter-exchange trunk lines over which voice frequency signaling currents are transmitted for supervision and for controlling the setting of automatic switches, repeaters associated with the incoming ends of said trunk lines for initially converting voice frequency signaling currents into corresponding direct current signals, repeaters associated with the outgoing ends of said trunk lines for initially converting direct current signals into corresponding voice frequency signaling currents, means for completing a connection to a called party through said trunk lines and associated repeaters in cascade, and means in said repeaters responsive to an answer by the called party for conditioning intermediate ones of said repeaters for straightthrough transmission of subsequent voice frequency signaling currents.

6. In a telephone system employing mixed systems of voice frequency and direct current signaling for controlling the selective setting of automatic switches, a repeater responsive to incoming direct current signals for transmitting corresponding voice frequency signals forward, said repeater being operative to test for an incoming voice frequency signal when seized and being operative in response to the presence of said voice frequency signal to condition itself for straight-through transmission.

'7. In a telephone system employing mixed systems of voice frequency and direct current signaling for controlling the setting of automatic switches, a repeater for converting incoming voice frequency signals into corresponding outgoing direct current signals to control succeeding auto-' matic switches-said repeater being operative to transmit a voice frequency testing signal forward after each direct current signal, a second repeater accessible to the succeeding automatic switches and operative to convert incoming direct current signals into corresponding outgoing voice frequency signals, said second repeater being operative in response to the reception of said testing signal to condition itself for straight-through transmission of voice frequency signals and to return a characteristic signal to said first repeater, and said first repeater being operative in response to said characteristic signal to condition itself for straight-through transmission of subsequent voice frequency signals.

8. A telephone system as claimed in claim 1 in which said consequent return signal comprises a momentary reversal of the direct current flowing over the speaking leads of the succeeding switches.

9. A telephone system as claimed in claim 2 in which said testing signal has the same frequency as a voice frequency supervisory signal and operates the same signal receiver in the sec ond repeater as said supervisory signal.

10. A telephone system as claimed in claim 1 in which said repeater is adapted to transmit the voice frequency testing signal for a predetermined minimum time interval after each direct current signal transmission.

11. A telephone system as claimed in claim 1 in which said repeater is adapted to terminate the transmission of the testing signal in response to reception of the consequent return signal.

12. A telephone system as claimed in claim 5 in which the operation of said last means is brought about by the forward transmission of an acknowledgment signal by the repeater at the originating end of the cascaded trunk lines in response to the reception of the called party answer signal.

CHARLES GILLINGS. CHARLES EDMUND BEALE. 

